Uncertainty about safety, steming from a lack of quantitative data on the effects of ultrasound on the conceptus, has limited the acceptance of ultrasonic techniques for prenatal diagnosis. These studies were designed to provide such data on the relationships between the level of exposure of the rat fetus to ultrasound and the effects produced. This includes exposure to graded intensities of ultrasound at either of two representative times of gestation (9 and 15 days) and under one of three well-defined conditions of frequency and pulse repetition rate representative of three major uses (Doppler, echographic, therapeutic). Individual implantation sites are insonated at 9 days and the fetuses evaluated at 20 days of gestation. Exposures to 0.71 or 3.2 MHz continuous wave ultrasound result in typical sigmoid intensity-mortality curves and increase malformation incidence at higher intensities. Average intensities of up to 4 W/cm2, 2.5 MHz pulsed ultrasound are not lethal but produce pathophysiological changes of the fetal heart. In studies at 15 days of gestation, one uterine horn is surgically removed and the fetuses of the other horn simultaneously exposed to a single intensity. The rats are allowed to undergo parturition and rear their litters. Early results indicate delayed parturition and a variety of neuromuscular deficits in the offspring.